1. Field of the Invention
The present invention relates to an optical apparatus such as a video camera, a still camera, a surveillance camera or the like.
2. Description of Related Art
Lens optical systems heretofore employed in the optical apparatuses of the above-stated kind are generally arranged, for example, as shown in FIG. 7. The lens optical system shown in FIG. 7 is a zoom lens composed of four lens groups with the fourth lens group which is in the rearmost position arranged to be movable for focusing in the direction of an optical axis. Referring to FIG. 7, the lens optical system includes a fixed front lens group 111, a variator lens group 112, a fixed lens group 113 and a focusing (compensator) lens group 114.
The lens optical system further includes a guide bar 133 provided for antirotation, a feed bar 134 arranged for moving the variator lens group 112, a fixed tube 135, a diaphragm unit 136 (inserted, in this case, perpendicular to the paper surface of the drawing), a stepping motor 137 employed as a focus motor, and an output shaft 138 of the stepping motor 137. The output shaft 138 is provided with a male screw 138a for moving the focusing lens group 114. The male screw 138a is in mesh with a female screw forming part 139 formed integrally with a moving frame 140 for moving the focusing lens group 114.
Guide bars 141 and 142 are arranged to guide the focusing lens group 114. A back plate 143 is arranged to position and retain the guide bars 141 and 142 in their positions. The optical system further includes a relay holder 144, a zoom motor 145, a speed reducer unit 146 arranged to reduce the speed of the zoom motor 145, and interlocking gears 147 and 148. The interlocking gear 148 is secured to the feed bar 134 for zooming.
The lens optical system shown in FIG. 7 operates as follows. When the stepping motor 137 is driven, the focusing lens group 114 is caused to move in the direction of the optical axis by screw feeding. When the zoom motor 145 is driven, the feed bar 134 is caused to rotate through the interlocking gears 147 and 148. The rotation of the feed bar 134 moves a lens frame 112a which is in screwed engagement with the feed bar 134, so that the variator lens group 112 held by the lens frame 112a is moved in the direction of the optical axis.
FIG. 8 shows by way of example the details of the diaphragm unit 136 used for the lens optical system. The diaphragm unit 136 is shown in FIG. 8 as viewed in the direction of the optical axis. Referring to FIG. 8, the diaphragm unit 136 includes an aperture part 208, a motor part 201, an output shaft (rotating shaft) 202, a diaphragm lever 203, projections 204 and 205 provided at the fore ends of the diaphragm lever 203, diaphragm blades 206 and 207, a diaphragm body 209, and guide parts 210 to 213 arranged to guide the diaphragm blades 206 and 207. The fore end projections 204 and 205 are inserted respectively into slots provided in the diaphragm blades 206 and 207. The diaphragm blades 206 and 207 are thus interlocked with the diaphragm lever 205. The aperture part 208 is formed jointly by the diaphragm blades 206 and 207. When the output shaft 202 rotates, the diaphragm blades 206 and 207 move upward and downward in opposite directions, as viewed in the drawing (the blade 207 moves downward while the blade 206 moves upward). The motions of the diaphragm blades 206 and 207 cause the size of the aperture of the aperture part 208 to vary accordingly. The motor part 201 serving as a drive source is mounted on the diaphragm body 209. The diaphragm body 209 is provided with the guide parts 210 to 213.
FIG. 9 shows in detail the structure of the motor part 201 of the diaphragm unit 136. A turning force is obtained by an ordinary known motor structure composed of a rotor magnet 215, coils 216 and 217 and a yoke (case) 214. The motor part 201 is also provided with a Hall element 218 for detecting the rotation of the motor part 201.
In addition to light quantity control by means of the diaphragm unit arranged as described above, a video camera or the like can perform light quantity control by the so-called shutter speed control means for controlling an electric charge storing time of an image sensor (CCD). FIG. 10(a) shows the electric charge storing time in relation to the field period of a television signal. In the case of the NTSC system, one field period which corresponds to {fraction (1/60)} sec is set to the electric charge storing time in its entirety. The lowest shutter speed is normally {fraction (1/60)} sec. The electric charge storing time can be shortened for a higher shutter speed, as shown in FIG. 10(b).
FIG. 11 shows in a block diagram a light quantity control arrangement conventionally adopted for a video camera. Referring to FIG. 11, a zoom lens is composed of lens groups 111 to 114 in the same manner as in the case of FIG. 7. A diaphragm unit 136 is arranged as shown in FIGS. 8 and 9. However, the diaphragm unit 136 is not limited to the arrangement having two diaphragm blades as in the case of FIGS. 8 and 9. An iris diaphragm which has more than two blades may be used for the diaphragm unit 136. A CCD 151 is employed as an image sensor. F-number detecting means 501 is generally arranged to detect the absolute rotating position of a rotor of the diaphragm unit 136 by means of a Hall element as shown in FIG. 9. A CPU 502 is arranged to control a driving action of each light quantity adjusting means in accordance with each program diagram which will be described later herein. The video camera shown in FIG. 11 further includes a CCD driving circuit 503, a camera circuit 504, a mode selecting means 505, a mode dial 506, a shutter speed designating means 507 and an aperture value designating means 508.
The camera circuit 504 is arranged to perform signal processing actions of varied kinds, such as an amplifying process, a gamma correction process, etc. Among the signals processed, a luminance signal is taken into the CPU 502. With the luminance signal taken in the CPU 502, the level of the luminance signal is checked to find whether the light quantity is apposite (a correct-exposure light quantity), or excessive (an over-exposure light quantity) or insufficient (an under-exposure light quantity). The CPU 502 then adjusts the light quantity according to the result of the check. For the light quantity adjustment, it is conceivable to control and adjust the diaphragm aperture diameter at the diaphragm unit 136 and the electric charge storing time, i.e., a shutter speed, at the CCD 151, as mentioned in the foregoing. Further, in a case where the light quantity is still insufficient, i.e., an under-exposure light quantity, with the diaphragm unit 136 fully opened to its maximum aperture position and the shutter speed set at its lowest speed, it is generally practiced to increase the gain of the video signal (a gain-up action) at the camera circuit 504. At the time of such light quantity adjustment, when the mode dial 506 is operated by the operator to select one of shooting (image-taking) modes of various kinds called an automatic mode, a sport mode, a portrait mode, etc., the manner of the light quantity adjustment, i.e., a program line, is changed according to the shooting mode thus selected. Further, when the mode dial 506 is set at a position for a manual mode, a value designated by the shutter speed designating means 507 or the aperture value designating means 508 is transmitted through the mode selecting means 505 to the CPU 502.
FIG. 12 shows combinations of aperture values and shutter speeds by which optimum light quantities can be obtained for different object luminances according to the shooting mode selected. Incidentally, the relation between the illuminance (luminance) and an exposure value EV cannot be exactly determined without having a value SV which corresponds to the film sensitivity of a silver-halide film. However, the graph of FIG. 12 and other graphs used for description hereinafter are set on the basis of the sensitivity of ordinary video cameras.
Referring to FIG. 12, a line which connects solid circles (xe2x97xaf) represents a maximum aperture priority program line. In the maximum aperture priority program line, the shutter speed is first increased accordingly as the luminance becomes brighter from a state of having EV7, {fraction (1/60)} sec and F1.4. In the case of FIG. 12, the upper limit of the shutter speed is set at {fraction (1/1000)} sec. The upper limit, however, may be set at a shutter speed higher than {fraction (1/1000)} sec. However, if the shutter speed is excessively high, the instability of a frequently moving object image would increase. The upper limit of the shutter speed is, therefore, preferably set according to the purpose of shooting. According to the program line connecting the solid circles (xe2x97xaf), after the luminance becomes brighter than EV11, the diaphragm unit 136 is driven so as to obtain an optimum light quantity. The program line connecting the solid circles (xe2x97xaf) is used for the case where the depth of field is to be made as shallow as possible so as to emphasize a blurring effect, for example, in the case of the portrait mode. Next, a program line which connects hollow triangles (xcex94) is used for the case where the so-called xe2x80x9cautomaticxe2x80x9d mode is set. In the case of FIG. 12, the program line for the automatic mode is arranged to perform light quantity control by driving the diaphragm unit 136 at exposure values between EV7 and EV12 and between EV16 and EV18 and by varying the shutter speed at exposure values between EV12 and EV16. This is because, in a case where the quality of image would be deteriorated by the diffraction of light to show flare with the modulation transfer function (MTF) lowered, if the aperture diameter is smaller than the diameter indicated by F8, for example, this program line is often provided for minimizing the adverse effect of image deterioration due to diffraction. Although the F-number in question is set at F8 in this case, this F-number varies with the size of image, the focal length of the lens optical system, etc.
A program line connecting hollow squares (xe2x96xa1) is arranged to be used for the case where a shutter speed of {fraction (1/250)} sec is to be used as much as possible. This program line applies to cases where the shutter speed of {fraction (1/250)} sec is selected under such light quantity control that gives priority to a shutter speed or where some high shutter speed is to be frequently used, like in the so-called sport mode.
It is a recent trend to use a smaller CCD, which causes the above-stated diffraction incurring F-number to shift to a brighter F-number, i.e., a smaller F-number, and to have a smaller image size, which causes the focal length to become shorter in obtaining the same angle of view and, as a result, causes the depth of field to become deeper to make it difficult to obtain an image of a shallow depth of field. Besides, the CCDs have recently come to have a higher degree of sensitivity. Therefore, the range of conditions obtainable according to the above-stated light quantity control methods of the prior art for obtaining images with desired effects, such as a blurring effect, has come to be limited.
For example, with the portrait mode obtained by the program line connecting the solid circles (xe2x97xaf), the diaphragm cannot be kept at its maximum (full-open) aperture position for an ordinary outdoor scene to which an exposure value between EV12 and EV15 applies.
The invention is directed to the solution of the above-stated problem of the prior art.
To attain the above-stated object, in accordance with one aspect of the invention, there is provided an optical apparatus, which comprises selecting means for selecting one image-taking mode from among a plurality of image-taking modes, a variable density element, and density control means for controlling density of the variable density element, wherein the density control means is capable of controlling the density of the variable density element in a plurality of density control modes and selects one of the plurality of density control modes according to the image-taking mode selected by the selecting means.
In accordance with another aspect of the invention, there is provided an optical apparatus, which comprises selecting means for selecting one image-taking mode from among a plurality of image-taking modes, a variable density element, density control means for controlling density of the variable density element in a plurality of density control modes, light quantity adjusting means for adjusting the quantity of passing light by moving diaphragm blades in a plurality of light quantity adjusting modes, and deciding means for deciding a combination of the density control mode of the density control means and the light quantity adjusting mode of the light quantity adjusting means according to the image-taking mode selected by the selecting means.